Tiptap gapcursor plugin does not work well with collaboration

[aonnikov]

When gapcursor is used, then collaborative plugins show runtime errors. Steps to reproduce:

1. Open a document
2. Add a table to the end of document
3. Move cursor below the table so gapcursor is shown
4. Click somewhere outside of editor

When leaving the editor, it shows the following runtime error:

In prosemirror demo the same scenaro works fine, so the problem is probably in Tiptap plugin or in our code

https://demos.yjs.dev/prosemirror/prosemirror.htm

[aonnikov]

The issue occurs because of empty field name being used. We need to consider changing empty fields to something else.